Note that you don't need to install Template Conditionals plugin if you want to use conditionals inside XThreads settings only. You need to install Template Conditionals plugin if you want to use conditionals in your templates.

HTML and CSS knowledge is strongly recommended; you should also be familiar with MyBB's template system. Because you can use conditionals in various XThreads settings, a knowledge about conditional structure is a plus point for you to create a more dynamic applications.

The following templates will be modified when the plugin is installed:

editpost and newthread - adds {$extra_threadfields} to the template, which just displays any added thread fields

showthread - inserts {$first_post} before {$posts}; in XThreads, it's possible to customise the first post differently from other posts (requires the forum option "Show first post on every showthread page" to be set)

forumdisplay_threadlist

insert {$nullthreads} after {$threads}; this is just for display "null threads" if thread grouping is used

insert {$sort_by_prefix} before the first option in the sorter listbox - this is to give the option to sort threads by thread prefix

from the above, some other sorting options are added below the Sort By: Views option in the listbox

forumdisplay_threadlist_sortrating - similar to the above, inserts a sort by number of ratings option to the sorter listbox

Unless stated otherwise, the general procedure for upgrades is to simply upload the new files, then visit your AdminCP. In there, you'll be presented with a notice to run the upgrade - just follow what it says from there on.

Standard installation procedures apply. Download and extract it.
After extraction, you'll need to upload the plugin to your forum. To do this, upload everything in the Upload folder to the forum's root directory; ensure that the inc/plugins/*.php files get uploaded to inc/plugins on the server.

Some things you may need to pay attention to:

If you have changed the location of your admin directory, you need to rename the admin folder in the package.

If you are not using English for the AdminCP, you may wish to rename the english folder to your language before uploading (if no translation is available)

You may need to CHMOD the uploads/xthreads_ul/ folder to 0777 after uploading the files (probably won't though).

XThreads is a powerful MyBB plugin which provides a flexible environment for administrators to perform various modifications. The basic idea is giving you the ability to define custom fields for your threads, and providing methods to give you a lot of flexibility with customising forums. It's really difficult to explain XThreads in a simple manner. You can somewhat think of it as a tool to allow admins to perform a higher level of customisation on their forum.

This is possible if we have one textarea only. So, if our XThreads applications only use one textarea only, and it doesn't use MyBB textarea, we can grab the MyBB editor and attach it to our textarea.This is not XThreads limitation, but this is MyBB Editor limitation.
To do this:

Edit the custom thread field, and set the Use Custom Input HTML setting to Yes.

Inside the textarea of the Input Field HTML setting, by default, there is a code like this:

You need to make changes to the relevant templates. Use {$GLOBALS['threadfields']['key']} in templates to reference that field. For example, if the Key of the custom thread field is myfield, then you need to use {$GLOBALS['threadfields']['myfield']} in templates to reference that field. This is slightly different for file inputs type.

For displaying it in each thread on the threadlist, generally, you need to edit your forumdisplay_thread template.

For displaying it on your showthread page, generally, you need to edit your showthread/postbit/postbit_classic template.

For displaying it on your search results page, generally, you need to edit your search_results_threads_thread template.

For displaying it on your portal announcements page, generally, you need to edit your portal_announcement template.